Diagnostic system for monitoring catalyst operation using arc length ratio

ABSTRACT

A method and system for diagnosing catalyst operation in an internal combustion engine having a two-bank, three EGO sensor structure includes determining the ratio of the arc length between the post-catalyst EGO sensor signal and the arc length of a pre-catalyst EGO sensor signal over a selected time period. If the exhaust bank is a one-sensor bank having only a post-catalyst EGO sensor and no pre-catalyst EGO sensor, the system uses the arc length from the pre-catalyst EGO sensor in the two-sensor bank to calculate the arc length ratio, thereby allowing calculation of two arc length ratios without two matched pairs of EGO sensors. The ratio indicates the efficiency of the catalyst and may be compared with calibratable or experimentally-determined thresholds to monitor converter efficiency over time.

TECHNICAL FIELD

[0001] The present invention is directed to a system for monitoringcatalyst operation in an internal combustion engine having a two-bankexhaust system. More particularly, the invention is directed to adiagnostic system that monitors catalyst efficiency by comparing signalsbetween a pre-catalyst EGO sensor and a post-catalyst EGO sensor in twodifferent banks.

BACKGROUND ART

[0002] To meet current emission regulations, automotive vehicles mustregulate the air/fuel ratio (A/F) supplied to the vehicles' cylinders soas to achieve maximum efficiency of the vehicles' catalysts. For thispurpose, it is known to control the air/fuel ratio of internalcombustion engines using an exhaust gas oxygen (EGO) sensor positionedin the exhaust stream from the engine. The EGO sensor provides feedbackdata to an electronic controller that calculates preferred A/F valuesover time to achieve optimum efficiency of a catalyst in the exhaustsystem. More particularly, the EGO sensor feedback signals are used tocalculate desired A/F ratios via a jumpback and ramp process, which isknown in the art.

[0003] It is also known to have systems with two EGO sensors in a singleexhaust stream in an effort to achieve more precise A/F control withrespect to the catalyst window. Normally, a pre-catalyst EGO sensor ispositioned upstream of the catalyst and a post-catalyst EGO sensor ispositioned downstream of the catalyst. Finally, in connection withengines having two groups of cylinders, it is known to have a two-bankexhaust system coupled thereto where each exhaust bank has its owncatalyst as well as its own pre-catalyst and post-catalyst EGO sensors.

[0004] It is known in the art to monitor the efficiency of a catalyst bydetermining the arc length ratio between signals generated bycorresponding pre-catalyst and post-catalyst EGO sensors in the sameexhaust stream and connected to the same catalyst. This type of systemis described in U.S. Pat. No. 5,899,062 to Jerger et al. and entitled“Catalyst Monitor Using Arc Length Ratio of Pre- and Post-CatalystSignals”, the disclosure of which is incorporated herein by reference.

[0005] Sometimes, in a two-bank, four-EGO sensor exhaust system, one ofthe pre-catalyst EGO sensors degrades. In other circumstances, it isdesirable to purposely eliminate one of the pre-catalyst EGO sensors ina two-bank system to reduce the cost of the system. In either event, itis desirable to be able to monitor the catalyst efficiency in the groupof cylinders coupled to the exhaust bank having only one operational EGOsensor by using the signals received from the three operational EGOsensors alone. However, known methods for catalyst diagnosis require amatched set of pre-catalyst and post-catalyst EGO sensors in each bank,such as in a one-bank, two EGO sensor system or in a two-bank, four EGOsensor system, so that the arc lengths between the correspondingpre-catalyst and post-catalyst sensors can be compared. Thus, for atwo-bank, three EGO sensor system, only the catalyst in the two EGOsensor exhaust bank will be monitored and diagnosed, while the catalystin the bank having only one operational EGO sensor will remainunmonitored.

[0006] There is a need for an improved system that can monitor theoperation of a catalyst in a one-sensor bank even though the catalystonly has one EGO sensor coupled to it.

SUMMARY OF THE INVENTION

[0007] Accordingly, the present invention is directed toward a newsystem and method for monitoring the operation of both catalysts in aninternal combustion engine having a group of cylinders coupled to twofunctioning EGO sensors (the “two-sensor bank”) and another group ofcylinders coupled to one functioning EGO sensor (the “one-sensor bank”).More particularly, the operation of the catalyst in the one-sensor bankis monitored and diagnosed based on a signal from a post-catalyst EGOsensor connected to the catalyst and a signal from a pre-catalyst EGOsensor in a different bank and connected to a different catalyst.

[0008] In a preferred embodiment of the invention, for a system that isa missing a pre-catalyst EGO sensor in the one-sensor bank, the signalfrom the pre-catalyst EGO sensor in the two-sensor bank is used tocalculate a diagnostic signal for the catalyst in the one-sensor bank.In essence, the invention assumes that a signal characteristic for thenon-existent pre-catalyst EGO sensor in the one-sensor bank would be thesame as the signal characteristic of the existing pre-catalyst EGOsensor in the two-sensor bank and calculates a diagnostic signal for thecatalyst in the one-sensor bank accordingly. The diagnostic signal canbe, for example, a ratio of the arc lengths between the post-catalystand pre-catalyst EGO sensor signals.

[0009] Once the arc length ratios are calculated, the ratios can becompared with calibratable or experimentally-generated ratios to monitorthe catalyst efficiency over time. As a result, the invention canmonitor and diagnose the operation of the catalysts in both theone-sensor bank and the two-sensor bank even though the one-sensor bankdoes not have a matched pair of EGO sensors.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 illustrates an internal combustion engine according to apreferred embodiment of the invention;

[0011]FIG. 2 is a block diagram representing a known two-bank exhaustsystem with each bank having pre-catalyst and post-catalyst EGO sensors;

[0012]FIG. 3 is a flowchart illustrating a known method in which the arclength ratio is calculated for a two-sensor bank.

[0013]FIG. 4 is a block diagram representing a two-bank exhaust systemwherein one bank has a pre-catalyst and a post-catalyst EGO sensor andthe other bank has only a post-catalyst EGO sensor; and

[0014]FIG. 5 is a flowchart illustrating the inventive method in whichthe arc length ratio is calculated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0015]FIG. 1 illustrates an internal combustion engine. Engine 200generally comprises a plurality of cylinders, but, for illustrationpurposes, only one cylinder is shown in FIG. 1. Engine 200 includescombustion chamber 206 and cylinder walls 208 with piston 210 positionedtherein and connected to crankshaft 212. Combustion chamber 206 is showncommunicating with intake manifold 214 and exhaust manifold 216 viarespective intake valve 218 and exhaust valve 220. As described laterherein, engine 200 may include multiple exhaust manifolds with eachexhaust manifold corresponding to a group of engine cylinders. Intakemanifold 214 is also shown having fuel injector 226 coupled thereto fordelivering liquid fuel in proportion to the pulse width of signal FPWfrom controller 202. Fuel is delivered to fuel injector 226 by aconventional fuel system (not shown) including a fuel tank, fuel pump,and fuel rail (not shown).

[0016] Conventional distributorless ignition system 228 providesignition spark to combustion chamber 206 via spark plug 230 in responseto controller 202. A first two-state EGO sensor 204 is shown coupled toexhaust manifold 216 upstream of catalyst 232. A second two-state EGOsensor 234 is shown coupled to exhaust manifold 216 downstream ofcatalyst 232. The upstream EGO sensor 204 provides a feedback signalEGO1 to controller 202 which converts signal EGO1 into two-state signalEGOS1. A high voltage state of signal EGOS1 indicates exhaust gases arerich of a reference A/F and a low voltage state of converted signal EGO1indicates exhaust gases are lean of the reference A/F. The downstreamEGO sensor 234 provides signal EGO2 to controller 202 which convertssignal EGO2 into two-state signal EGOS2. A high voltage state of signalEGOS2 indicates that the engine is running rich, and a low voltage stateof converted signal EGO1 indicates that the engine is running lean.Controller 202 is shown in FIG. 1 as a conventional microcomputerincluding: microprocessor unit 238, input/output ports 242, read onlymemory 236, random access memory 240, keep alive memory 241 and aconventional data bus.

[0017]FIGS. 2 and 4 schematically illustrate different embodiments of atwo-bank exhaust system to be used in the present invention. FIG. 2shows a known two-bank, four EGO-sensor exhaust system. As illustratedin FIG. 2, exhaust gases flow from first and second groups of cylindersof engine 12 through a corresponding first exhaust bank 14 and secondexhaust bank 16. Engine 12 is the same as or similar to engine 200 inFIG. 1. Exhaust bank 14 includes pre-catalyst EGO sensor 18, catalyst20, and post-catalyst EGO sensor 22. Exhaust bank 16 includespre-catalyst EGO sensor 24, catalyst 26 and post-catalyst EGO sensor 28.The pre-catalyst EGO sensors, catalysts, and post-catalyst EGO sensorsin FIG. 2 are the same as or similar to pre-catalyst EGO sensor 204,catalyst 232, and post-catalyst EGO sensor 234 in FIG. 1.

[0018] In operation, when exhaust gases flow from engine 12 throughexhaust bank 14, pre-catalyst EGO sensor 18 senses the emissions levelin the exhaust gases passing through bank 14 before they enter catalyst20 and provides feedback signal EGO1 a to controller 202. After theexhaust gases pass through catalyst 20, post-catalyst EGO sensor 22senses the emissions level in the exhaust gases after they exit thecatalyst 20 and provides feedback signal EGO1 b to controller 202. Withrespect to exhaust bank 16, pre-catalyst EGO sensor 24 senses theemissions level in the exhaust gases passing through bank 16 before theyenter catalyst 26 and provides feedback signal EGO2 a to controller 202.After the exhaust gases pass through catalyst 26, post-catalyst EGOsensor 28 senses the emissions level in the exhaust gases after theyexit catalyst 26 and provides feedback signal EGO2 b to controller 202.Then the exhaust gases are joined at junction 29 before being expelledfrom the system 10, though the disclosed invention is equally applicableto a system wherein the exhaust banks are kept separate throughout theentire system. Controller 202 uses feedback signals EGO1 a, EGO1 b, EGO2a, and EGO2 b, which reflect the current operating conditions of thecatalysts 20, 26, to calculate the arc length ratios for diagnosingcatalyst operation. The controller shown in FIG. 2 is the same as orsimilar to controller 202 shown in FIG. 1.

[0019] Catalyst operation can be monitored by comparing selected signalcharacteristics, such as the arc length, of the signals from thepre-catalyst and post-catalyst EGO sensors connected to that catalyst.Although the present application focuses on calculating a catalystdiagnostic signal based on the arc lengths of the EGO sensor signals,any signal characteristic can be used as long as one signal is from apre-catalyst EGO sensor and the other signal is from a post-catalyst EGOsensor, even if the sensors are in different exhaust banks. One way inwhich the arc length ratios are calculated for a two-sensor bank isexplained in U.S. Pat. No. 5,899,062, which is incorporated herein byreference. A flowchart of the known calculation process is shown in FIG.3. Because each catalyst 20, 26 is coupled to both a pre-catalyst EGOsensor 18, 22 and a post-catalyst EGO sensor 24, 28 in each bank 14, 16,the same process is used to calculate the arc length ratios formonitoring each catalyst 20, 26. In this case, the system samples boththe pre-catalyst EGO sensor signals and post-catalyst EGO sensor signals32 and then determines incremental signal arc lengths 34 from thesamples. An instantaneous ratio is calculated 36 from the incrementalarc lengths, preferably by dividing the incremental arc length of thepost-catalyst signal by the incremental arc length of the pre-catalystsignal for a given catalyst. The system then sums the incremental arclengths of each signal 38 from the EGO sensors to obtain an estimate ofthe line integral for a particular signal segment and calculates anaccumulated arc length ratio based on the summed arc lengths 40. Theinstantaneous and accumulated arc length ratios are then stored inmemory 42 and used to monitor the efficiency of the catalyst 44. Forexample, the arc length of the post-catalyst signal with respect to thearc length of the pre-catalyst signal will increase as the catalyst agesand becomes less efficient.

[0020]FIG. 4 illustrates a two-bank exhaust system similar to that shownin FIG. 2, except that the pre-catalyst EGO sensor in one of the exhaustbanks 36 is missing. Specifically, FIG. 4 illustrates that exhaust gasesexpelled from engine 32 pass through exhaust banks 34 and 36. In bank34, the emissions level of the exhaust gases is sensed by pre-catalystEGO sensor 38 before entering catalyst 40, and feedback signal EGO1 a isprovided to controller 202. After the exhaust gases exit catalyst 40,the emissions level is sensed by post-catalyst EGO sensor 42, andfeedback signal EGO2 a is provided to controller 202. With respect toexhaust bank 36, the exhaust gases expelled by engine 32 enter catalyst44. After the exhaust gases exit catalyst 44, their oxygen content issensed by post-catalyst EGO sensor 46, and feedback signal EGO2 b isprovided to controller 202. Then the exhaust gases are joined atjunction 48 before being expelled from the system 30, though thedisclosed invention is equally applicable to a system wherein theexhaust banks are kept separate throughout the entire system.

[0021]FIG. 5 is a flowchart illustrating the arc length ratiocalculation process 50 according to the present invention. Because oneof the banks 36 does not have a pre-catalyst EGO sensor, the processmust also include the step of checking whether a pre-catalyst sensor isconnected to the catalyst being monitored 56. If both pre-catalyst andpost-catalyst EGO sensors are coupled to the catalyst (i.e. the catalystis in a two-sensor bank), then the system continues calculating the arclength ratio in the known manner explained above 60, 62, 64, 66. If,however, the catalyst only has a post-catalyst EGO sensor coupled to itwith no corresponding pre-catalyst EGO sensor (i.e. the catalyst is in aone-sensor bank, as shown in FIG. 4), the invention uses the arc lengthof the pre-catalyst sensor signal in the two-sensor bank of the enginefor the arc ratio calculation in the one-sensor bank 58. In short, theinvention assumes that the arc length of the missing pre-catalyst EGOsensor in the one-sensor bank would be the same as the arc length of theexisting pre-catalyst EGO sensor in the two-sensor bank. This allowscalculation of the arc ratios for both catalysts with only threemeasured arc lengths instead of the four arc lengths that areconventionally required in known methods. The arc length ratiocalculations according to the present invention would therefore be asfollows:

Arc_ratio_(—)1=Arc_length12/Arc_length11

Arc_ratio_(—)2=Arc_length22/Arc_length11

[0022] where:

[0023] Arc_ratio_(—)1: arc ratio, two-sensor bank

[0024] Arc_ratio_(—)2: arc ratio, one-sensor bank

[0025] Arc_length11: pre-catalyst sensor signal arc length, two sensorbank

[0026] Arc_length12: post-catalyst sensor signal arc length, two sensorbank

[0027] Arc_length22: post-catalyst sensor signal arc length, one sensorbank

[0028] Note that although the present invention was described in termsof a two-bank, three-EGO sensor system, as shown in FIG. 4, it iscontemplated and should be understood that this invention can also beused in connection with a well-known two-bank four-EGO sensor system, asshown in FIG. 2, for purposes of compensating for a degradedpre-catalyst EGO sensor in one of the banks. In such a system, knownmethods, such as the method described in U.S. Pat. No. 5,899,062, can beused to monitor the catalysts in both banks while all four EGO sensorsare operating properly. In the event that one of the pre-catalyst EGOsensors degrades, and if the degradation is detected by the system, theinvention compensates for the degraded EGO sensors by conducting the arcratio calculation using only three arc length measurements.

[0029] It should be understood that various alternatives to theembodiments of the invention described herein may be employed inpracticing the invention. It is intended that the following claimsdefine the scope of the invention and that the method and apparatuswithin the scope of these claims and their equivalents be coveredthereby.

What is claimed is:
 1. A system for monitoring catalyst operation in anengine having a first catalyst and a second catalyst, comprising: afirst EGO sensor coupled to the first catalyst at a post-catalystlocation, the first EGO sensor generating a first signal; a second EGOsensor coupled to a second catalyst at a pre-catalyst location, thesecond EGO sensor generating a second signal; a controller coupled tosaid first and second EGO sensors for calculating a first diagnosticsignal corresponding to the operation of the first catalyst, wherein thecontroller calculates the first diagnostic signal from the first andsecond arc lengths.
 2. The system of claim 1 , wherein the first signalhas a first arc length, the second signal has a second arc length, andthe first diagnostic signal is a first arc length ratio of the first andsecond arc lengths.
 3. The system of claim 2 , wherein the first arclength ratio for the first catalyst is the first arc length divided bythe second arc length.
 4. The system of claim 2 , further comprising athird EGO sensor coupled to the second catalyst at a post-catalystlocation, the third EGO sensor generating a third signal, wherein thecontroller calculates a second diagnostic signal for the second catalystfrom the second and third signals.
 5. The system of claim 4 , whereinthe third signal has a third arc length and the second diagnostic signalis a second arc length ratio of the second and third arc lengths.
 6. Thesystem of claim 5 , wherein the first arc length ratio for the firstcatalyst is the first arc length divided by the second arc length, andwherein the second arc length ratio for the second catalyst is the thirdarc length divided by the second arc length.
 7. A method for monitoringcatalyst operation in an engine having a first catalyst and a secondcatalyst, comprising: generating a first signal from a first EGO sensorcoupled to the first catalyst at a post-catalyst location; generating asecond signal from a second EGO sensor coupled to a second catalyst at apre-catalyst location; calculating at least a first diagnostic signalcorresponding to the operation of the first catalyst from the first andsecond signals.
 8. The method of claim 7 , wherein the first signal hasa first arc length, the second signal has a second arc length, and thefirst diagnostic signal is a first arc length ratio of the first andsecond arc lengths.
 9. The method of claim 8 , wherein the arc lengthratio for the first catalyst is the first arc length divided by thesecond arc length.
 10. The method of claim 7 , wherein the calculatingstep comprises the steps of: determining first signal and second signalincremental arc lengths of the first and second signals, respectively;and calculating a first instantaneous arc length ratio from the firstand second signal incremental arc lengths obtained in the determiningstep.
 11. The method of claim 10 , wherein the first instantaneous arclength ratio for the first catalyst is the first signal incremental arclength divided by the second incremental signal arc length.
 12. Themethod of claim 11 , wherein the calculating step further comprises thesteps of: summing the first and second signal incremental arc lengths ofthe first and second signals to obtain first and second accumulated arclengths, respectively; and calculating a first accumulated arc lengthratio corresponding to the first catalyst from the first and secondaccumulated arc lengths.
 13. The method of claim 12 , wherein the firstaccumulated arc length ratio for the first catalyst is the first signalaccumulated arc length divided by the second accumulated signal arclength.
 14. The method of claim 7 , further comprising the steps of:generating a third signal from a third EGO sensor coupled to the secondcatalyst at a post-catalyst location, and calculating at least a seconddiagnostic signal corresponding to the operation of the second catalystfrom the second and third signals.
 15. The method of claim 14 , whereinthe first, second, and third signals have first, second and third arclengths, respectively, and wherein the first diagnostic signal is afirst arc length ratio of the first and second arc lengths and thesecond diagnostic signal is a second arc length ratio of the second andthird arc lengths.
 16. The method of claim 14 , wherein the arc lengthratio for the first catalyst is the first arc length divided by thesecond arc length, and wherein the arc length ratio for the secondcatalyst is the third arc length divided by the second arc length. 17.The method of claim 14 , wherein the calculating step comprises thesteps of: determining first signal, second signal and third signalincremental arc lengths of the first, second and third signals,respectively; calculating a first instantaneous arc length ratiocorresponding to the first catalyst from the first and second signalincremental arc lengths obtained in the determining step; andcalculating a second instantaneous arc length ratio corresponding to thesecond catalyst from the second and third incremental arc lengthsobtained in the determining step.
 18. The method of claim 17 , whereinthe first instantaneous arc length ratio for the first catalyst is thefirst signal incremental arc length divided by the second incrementalsignal arc length, and wherein the second instantaneous arc length ratiofor the second catalyst is the third incremental arc length divided bythe second incremental arc length.
 19. The method of claim 18 , whereinthe calculating step further comprises the steps of: summing the first,second and third signal incremental arc lengths of the first, second andthird signals to obtain first, second and third accumulated arc lengths,respectively; calculating a first accumulated arc length ratiocorresponding to the first catalyst from the first and secondaccumulated arc lengths; and calculating a second accumulated arc lengthratio corresponding to the second catalyst from the second and thirdaccumulated arc lengths.
 20. The method of claim 19 , wherein the firstaccumulated arc length ratio for the first catalyst is the first signalaccumulated arc length divided by the second accumulated signal arclength, and wherein the second accumulated arc length ratio for thesecond catalyst is the third accumulated arc length divided by thesecond accumulated arc length.